.mp4 can be accessed normally outside blender but I can't make the sound work on blender vse preview or even render a video with audio
You will find folks giving you advice and make you jump through hoops and walk through fire just to get your video to play in blender, but the bottom line is that Blenders VSE is a very inefficient and antiquated program. Why put yourself through a poor experience when there are better, friendlier and responsive programs out there? Transcoding shouldn't be necessary in today's software and hardware. Try shutcut, DaVinci resolve free, openshot, media composer first, premiere, etc etc. A search in your favorite search engine for free or cheap video editing sotware will give you plenty of choices. Be happy, stay creative. Don't be bogged down by the tools. Make the tools work for you, not the other way around. Blenders is great for creating 3d, but not a great video editor.